1. Field of the Invention
The embodiments of the invention relate to amplifiers and, more particularly, to power up processing and power down processing of an amplifier.
2. Description of Related Art
Integrated circuit amplifiers perform a variety of tasks including the use of an amplifier to drive low impedance loads. One particular type of output amplifier is used to drive a low frequency signal to a low impedance load. One such example is an output amplifier used to drive an audio signal to a low impedance load such as a headphone. In a typical arrangement, to pass low frequency signals to the low impedance load, a direct current (DC) blocking capacitor may be employed between the amplifier and the load in order to block the DC voltage. In order to ensure reasonable bass response for an audio amplifier, the capacitance value of the blocking capacitor tends to be large. For example, in order to pass audio signals of 20 Hz or greater to a low impedance headphone load (approximately 16 Ohms), it is not unreasonable to find DC blocking capacitors having a value in the approximate range of 490 microfarads (μF).
When large capacitors are utilized for DC blocking purposes, a substantial amount of time may be required to charge the capacitor to its operating potential when the circuit is first activated. If the charging current is not limited, there is a possibility that charging rate may exceed a voltage value that will cause an audible pop and click to be heard in the headphone.
For example, assuming that a 1 millivolt step change may cause a noticeable pop and click in the headphone, the maximum charge current step function allowed for a 16 Ohm load would be approximately 62 microamps (1 mV/16 Ohm=62 μA). That is, the charging current may be increased without being audible, but it should be incrementally increased (e.g. start at 62 μA, wait for a time period, increase current to 124 μA, wait for a time period, increase to 186 μA, etc.). Each step increase causes an approximate 1 mV initial step on the load, but then increases the charging ramp rate dV/dt=I/C. Assuming the use of a 490 μF capacitor and assuming that the capacitor would be charged to a voltage of around 1 Volt at the incrementally increased value of 62 μA, it would take approximately 7.6 seconds for the capacitor to be fully charged. This is a considerable amount of time to wait once the amplifier is initially powered. Many electronic devices that would employ a headphone, such as MP3 (MP3 is an audio standard under the Moving Picture Experts Group or MPEG) players, such lag time is not desirable, since the user would need to wait until the amplifier powers up to its operating level.
In order to ensure fairly short charge time of the blocking capacitor for audio applications, one technique uses a number of current sources at the output to charge the DC blocking capacitor to provide the incrementally increased current noted above. In this technique, additional current sources are switched in parallel to increase the charging current as the capacitor begins to charge. With each incremental current source switched in, the charging current is increased incrementally in a step function as explained above. Specific time intervals are designated for switching in the additional current(s) to reduce the time to fully charge the blocking capacitor, but at the same time to have an acceptable anti-pop (anti-click) performance.
A similar problem is encountered when the amplifier is powered down. The fully charged DC blocking capacitor now needs to discharge at a rate that should not cause a pop and click at the output. In order to obtain acceptable anti-pop (anti-click) performance for powering down the headphone, a decremental current response is typically obtained by switching in parallel resistors at specific time intervals.
A drawback of the current source-resistor switching in approach is that a number of current sources and resistors are employed with the output stage of the amplifier, adding extra circuit components to the amplifier. Furthermore, the incremental stepping-up of the current or the decremental stepping-down of the current is not continuous and may cause coarse granularity of the current adjustment to charge and discharge the DC blocking capacitor.